Overtopping

Overtopping is the flow of water that passes over the crest or boundary of a barrier, structure, or landform. It commonly occurs on hydraulic structures such as dams, levees, weirs, and seawalls, as well as on natural or engineered coastal features like dunes and revetments when storms, high tides, or extreme inflows push water above the crest.

In dam safety, overtopping happens when upstream water levels exceed crest freeboard or when spillways are inadequate to pass the discharge. Uncontrolled overtopping can erode the crest and foundation, induce piping and seepage, and potentially lead to a breach if the water flow persists or increases. In coastal engineering, wave overtopping refers to waves that run up and over a coastal defense, delivering water onto the land behind the structure. Repeated overtopping can cause flooding, scour, structural damage to defenses and adjacent infrastructure, and adverse ecological effects.

Causes of overtopping include extreme hydrological events (floods), storm surge, high-energy sea states, rising water levels, insufficient crest height, and degradation or damage to the protective structure. Overtopping can be episodic or sustained, with the rate and depth of overtopping determining the level of risk to the protected area.

Mitigation approaches focus on reducing crest overflow and runoff energy. Methods include raising crest height, providing spillways or overflow channels, implementing energy-dissipation devices, improving drainage and toe protection, and ongoing maintenance and monitoring. Overtopping is assessed through design studies, field measurements, and numerical modeling to ensure that safety and flood defense objectives are met.